An in-mold foaming molded product which is obtained by (i) introducing a polypropylene resin expanded particles into a mold and (ii) thermally molding the expanded particles with steam has characteristics such as freedom of shape, lightness in weight, and heat insulation efficiency which are advantages of the in-mold foaming molded product. The in-mold foaming molded product prepared from the polypropylene resin expanded particles is superior in chemical resistance, heat resistance, and a distortion recovery rate after compression, in comparison with polystyrene resin expanded particles. In addition, the in-mold foaming molded product prepared from the polypropylene resin expanded particles is superior in dimensional accuracy, heat resistance, and compressive strength in comparison with polyethylene resin expanded particles. Because of those characteristics, the in-mold foaming molded product prepared from the polypropylene resin expanded particles is variously used for heat insulators, buffer packaging materials, automobile interior part materials, core material for automobile bumpers, and the like.
However, in order to prepare the in-mold foaming molded product by fusing the polypropylene resin expanded particles with each other in a mold, it is necessary to heat the polypropylene resin expanded particles at a high temperature, i.e., under a high steam pressure in comparison with cases of the polystyrene resin expanded particles and the polyethylene resin expanded particles. Therefore, this method of preparing the in-mold foaming molded product has such disadvantages in that this method requires a mold and a molding machine tolerable against a high pressure, and high steam cost for molding.
Most of molding machines for in-mold foaming molding of the polypropylene resin expanded particles have an upper limit of pressure resistance of up to about 0.4 MPa·G (gauge pressure). The polypropylene resin expanded particles to be used for the in-mold foaming molding are prepared from a resin which can be molded under the pressure up to the upper limit of the pressure resistance of the molding machines. Generally, the polypropylene resin expanded particles made from a propylene random copolymer having a melting point of about 140° C. to 150° C. are used.
However, due to a recent steep rise in fuel price etc., there is a demand for molding an in-mold foaming molded product at a lower temperature, i.e., with a lower steam pressure. Further, in a case where the in-mold foaming molded product is prepared using a mold having a complicated shape or a large mold, there is a risk that the expanded particles are not fused well to each other in some parts in the molding. If the molding is performed with a higher mold heating steam pressure in order to perform the fusing sufficiently in such parts, the expanded particles tend to deform or shrink. Accordingly, it is desired that a range of mold heating steam pressure (also referred to as a range of heating condition) for molding is improved to be wider than a conventional one.
In order to solve the aforementioned problems, a propylene.1-butene random copolymer, a propylene.ethylene.1-butene random ternary copolymer, prepared by using a Ziegler polymerization catalyst, (Patent Literature 1 and Patent Literature 2), and a PP homopolymer or a propylene-ethylene random copolymer, prepared by using a metallocene polymerization catalyst, are proposed as a resin which has a low melting point and has a higher rigidity than other resins at the same melting point.
However, a propylene random copolymer containing a 1-butene comonomer, which is prepared by using a Ziegler polymerization catalyst, has a limit in reducing its melting point. In specific, melting points of commercially available propylene random copolymers are about 130° C. at lowest.
On the other hand, a propylene.ethylene random copolymer prepared by using a metallocene polymerization catalyst can be prepared to have a lower melting point, specifically, of 130° C. or less.
In order to achieve in-mold foaming molding at low heating temperature, Patent Literature 3 proposes that polypropylene resin expanded particles made from a polypropylene resin having a resin melting point of 115° C. to 135° C. and an Olsen flexural modulus of 500 MPa or higher. The resin used therein is prepared from partially a propylene.ethylene.1-butene random ternary copolymer and mainly a propylene.ethylene.random copolymer prepared by using a metallocene polymerization catalyst. Melting points of the resin in Examples of Patent Literature 3 are in a range of 120° C. to 134° C. Considering this, the technique of Patent Literature 3 surely achieves the in-mold foaming molding at a low heating temperature. However, the technique of Patent Literature 3 should be improved in terms of a range of mold heating steam pressure allowing use of a high temperature.
As a technique for widening the range of mold heating steam pressure, Patent Literature 4 proposes polypropylene resin pre-expanded particles made from a mixture of two polypropylene resins having a melting point difference of 15° C. or more but 30° C. or less. However, it is necessary to mold the polypropylene resin pre-expanded particles at a mold heating temperature of 140° C. or more, that is, the pre-expanded particles cannot be said as being moldable at a low temperature.
Further, Patent Literature 5 proposes that polypropylene resin expanded particles having a crystal structure which indicate a main endothermic peak exhibiting an endothermic peak calorific value of 70 to 95% of the total endothermic peak calorific value and having an apex temperature of 100 to 140° C. and two or more endothermic peaks on the high-temperature sides of the main endothermic peak on the first DSC curve obtained when the polypropylene resin expanded particles are heated from normal temperature to 200° C. at a temperature rise rate of 2° C./min by heat flux differential scanning calorimeter. The polypropylene resin expanded particles are made from a mixture of (i) a propylene.ethylene random copolymer having a low melting point and being prepared by using a metallocene polymerization catalyst and (ii) a resin having a high melting point such as a propylene homopolymer and being prepared by using a Ziegler polymerization catalyst. Examples of Patent Literature 5 describe that a preferable in-mold foaming molded product can be prepared with a heating steam pressure less than 0.2 MPa·G, which pressure is lower than conventional polypropylene resin expanded particles. However, in Patent Literature 5, two component resins have an excessively large melting point difference therebetween, and therefore it is expected that the expanded particles tend to have a cell structure disturbed to have open-cell foams.